cyclin
Summary
Cyclins are regulatory proteins that bind and activate cyclin-dependent kinases (CDKs) to drive progression through the cell cycle. Their levels oscillate periodically, rising and falling at specific phases to control checkpoints (G1/S, G2/M). Dysregulation of cyclins (e.g., Cyclin D overexpression) is implicated in oncogenesis.
Detail
Cyclins are a family of proteins whose concentration fluctuates cyclically throughout the cell cycle, in contrast to CDKs which are present at constant levels but require cyclin binding for activity. Each cyclin-CDK complex phosphorylates specific substrates to promote transition through cell cycle checkpoints:
- Cyclin D-CDK4/6: drives G1 progression by phosphorylating retinoblastoma protein (Rb), releasing E2F transcription factors to promote S-phase gene expression. - Cyclin E-CDK2: regulates G1/S transition. - Cyclin A-CDK2: active during S phase. - Cyclin B-CDK1 (MPF, maturation-promoting factor): drives G2/M transition and mitosis.
Cyclin levels are controlled by synthesis and degradation via the ubiquitin-proteasome pathway (APC/C - anaphase promoting complex targets cyclin B for destruction to allow exit from mitosis). CDK inhibitors (CKIs) such as p21 and p27 (regulated by p53) and INK4 family (p16) inhibit cyclin-CDK complexes, providing checkpoint control and tumor suppression.
Clinical significance: Cyclin D1 (CCND1) overexpression, often due to t(11;14) translocation placing CCND1 under IgH enhancer control, is characteristic of mantle cell lymphoma. Cyclin D1 amplification is also seen in breast and other cancers. Loss of CDK inhibitors (e.g., p16/CDKN2A mutations) or Rb loss leads to unchecked cyclin-CDK activity and uncontrolled proliferation. This pathway is targeted therapeutically by CDK4/6 inhibitors (palbociclib, ribociclib) used in HR+/HER2- breast cancer.
Understanding cyclins is essential for understanding cell cycle checkpoints, tumor suppressor gene function (Rb, p53), and mechanisms of targeted cancer therapy.
Sources
- First Aid for the USMLE Step 1
- Robbins Basic Pathology
- Molecular Biology of the Cell (Alberts)
- Kaplan USMLE Step 1 Biochemistry/Cell Biology
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